During operation of an electrolytic cell for aluminum production, the cathode block becomes damaged and will need to be replaced. This is a normal procedure which will typically take place after several years of operation. During rebuilding of an electrolytic cell which may typically take up to about one month and significant resources, the electrolytic cell is out of production. Regardless of the reason for the start-up of a cell, whether a rebuild or a new cell start-up, it is of interest to minimize the impact of any down time and to put a cell into operation as soon as possible.
Prior to putting an electrolytic cell into operation, the cathode block must be pre-heated, typically to a temperature of from about 800 to 900° C. This may be done in various ways, including for example, applying a granular conductive material like coke or graphite in rounds on the surface of the cathode beneath the anodes and applying power to the anodes to thereby transmit electrical current to the cathode block. The granular conductive material applied between the cathode and the anodes is often referred to as a contact resistance material. Coke or graphite may be selected to obtain the desired electrical resistance of a contact material so as to deliver more or less heat to the electrolytic cell.
In U.S. Pat. No. 7,485,215, a process is described in which the periphery of the electrolytic cell is filled with crushed electrolyte bath material and sodium carbonate. In addition, rock wool is applied against the upper surface and the outer surfaces of the anodes as well as over the central corridor of the electrolytic cell in order to minimize heat losses from the electrolytic cell during pre-heating of the cathode block. The electrolytic cell is then energized so as to cause an electric current to flow between the anodes and the cathode block.
Once the cathode is pre-heated, which may take place over a period of 36 to 48 hours, sufficient molten bath taken from other so-called donor cells which are in operation, is added to the electrolytic cell for immersing the anodes and to raise the anodes to operational levels without creating any open electrical circuits. Molten bath obtained from donor cells is normally used despite the disturbances arising from having to melt crushed bath from donor cells. However, this is not always an option, particularly in “Greenfield” operations where donor cells are unavailable at least until some electrolytic cells are put into operation. The molten electrolyte bath becomes the conductor material between the anode and the cathode so the heat up phase continues up to fourteen to thirty two hours and finally, after that heat-up phase is complete, molten aluminum metal is added to cover the cathode surface beneath the molten electrolyte bath. At this stage, a solid crust is formed on top of the bath and the anodes may be covered with the usual additions of alumina, solid granulated bath, and additives such as AlF3 and calcium to thermally isolate the cell. Normal operation can begin with an optimal heat balance of the cell giving the opportunity to reduce energy input.
In such a traditional cell start-up, five to twelve tons of molten electrolyte bath from about ten donor cells are required, depending on the size of the electrolytic cell. This is a very labor-intensive operation which not only is time consuming but also monopolizes use of the crane to siphon and transport molten electrolyte bath from donor cells to the start-up cell. This can be a problem in an operating plant where the same crane is also needed to siphon metal and for regular anode changing operations. In addition to the labor involved with liquid bath transportation, more care is required to maintain the donor cells in operation which is particularly challenging when starting up a “Greenfield” operation.
Previously, in some “Greenfield” operations, attempts were made to start-up a new cell by applying a thin layer of cryolite to the upper surface of the cathode block around the coke up to a height of about 5 to 10 centimeters (1.97 to 3.94 inches), in order to insulate the area surrounding the anodes and to direct the heat generated from the coke towards the cathode block. These early attempts at dry cell start-ups were fraught with problems and were subsequently abandoned by the aluminum smelting community. As soon as any molten pools of cryolite would form, the molten material would settle in low areas of the cathode and subsequently freeze if the underlying cathode surface was not sufficiently preheated. Severe start-up problems would occur if the molten cryolite had settled beneath an anode, thereby electrically insulating the cathode and causing the anode to short-circuit. The resulting current distribution in the cell became so electrically unstable that aluminum refineries resorted to such dry start-up procedures only when absolutely necessary and only with the support of an expert team of operators and management personnel.